Add shared test cases with vim-svelte-plugin

Name cases for scenarios also in vim-svelte-plugin as shared-*, adding
one for multiline tags. test/shared.sh lists shared cases in only one
of the plugins, as a warning in a separate job on GitHub Actions.

test/cases/shared-undo.vue

@@ -0,0 +1,3 @@
+<template>
+  <div>x</div>
+</template>